This invention relates to organic electronic devices and in particular buffer layers for organic light emitting diodes (OLEDs).
Organic electronic devices are articles that include layers of organic materials, at least one of which can conduct an electric current. An example of an organic electronic device is an organic light emitting diode (OLED). OLEDs, sometimes referred to as lamps, are desirable for use in electronic media because of their thin profile, low weight, and low driving voltage, i.e., less than about 20 volts. OLEDs have potential use in applications such as backlighting of graphics, pixelated displays, and large emissive graphics.
OLEDs typically consist of an organic light emitter layer and additional organic charge transport layers on both sides of the emitter, all of which are sandwiched between two electrodes: a cathode and an anode. The charge transport layers comprise an electron transporting layer and a hole transporting layer. Charge carriers, i.e., electrons and holes, are injected into the electron and hole transporting layers from the cathode and anode, respectively. Electrons are negatively charged atomic particles and holes are vacant electron energy states that behave as though they are positively charged particles. The charge carriers migrate to the emitter layer, where they combine to emit light.
FIG. 1 illustrates a type of organic light emitting diode. The diode comprises a substrate 12, a first electrode (anode) 14, a hole transporting layer 16, a light emitting layer 18, an electron transporting layer 20, and a second electrode (cathode) 22.
Substrate 12 may be transparent or semi-transparent and may comprise, e.g., glass, or transparent plastics such as polyolefins, polyethersulfones, polycarbonates, polyesters, and polyarylates.
Anode 14 is electrically conductive and may be optically transparent or semi-transparent. Suitable materials for this layer include indium oxide, indium-tin oxide (ITO), zinc oxide, vanadium oxide, zinc-tin oxide, gold, copper, silver, and combinations thereof.
An optional hole injecting layer (not shown) may accept holes from anode 14 and transmit them to hole transporting layer 16. Suitable materials for this layer include porphyrinic compounds, e.g., copper phthalocyanine (CuPc) and zinc phthalocyanine.
Hole transporting layer 16 facilitates the movement of holes from anode layer 14 to emitter layer 18. Suitable materials for this layer include, e.g., aromatic tertiary amine materials described in U.S. Pat. Nos. 5,374,489 and 5,756,224, (both incorporated by reference) such as 4,4xe2x80x2,4xe2x80x3-tri(N-phenothiazinyl) triphenylamine (TPTTA), 4,4xe2x80x2,4xe2x80x3-tri(N-phenoxazinyl) triphenylamine (TPOTA), N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine (TPD), and polyvinyl carbazole.
Emitter layer 18 comprises an organic material capable of accommodating both holes and-electrons. In emitter layer 18, the holes and electrons combine to produce light. Suitable materials for this layer include, e.g., tris(8-hydroxyquinolinato)aluminum (AlQ). The emission of light of different colors may be achieved by the use of different emitters and dopants in the emitter layer as described in the art (see C. H. Chen, J. Shi, and C. W. Tang xe2x80x9cRecent Developments in Molecular Organic Electroluminescent Materialsxe2x80x9d, Macromolecular Symposia 1997 125, 1-48).
Electron transporting layer 20 facilitates the movement of electrons from cathode 22 to emitter layer 18. Suitable materials for this layer include, e.g., AlQ, bis(10-hydroxy-benzo(h)quinolinato) beryllium, bis(2-(2-hydroxy-phenyl)-benzolthiazolato) zinc and combinations thereof.
An optional electron injecting layer (not shown) may accept electrons from the cathode 22 and transmit them to the emitter layer 18. Suitable materials for this layer include metal fluorides such as LiF, CsF, as well as SiO2, Al2O3, copper phthalocyanine (CuPc), and alkaline metal compounds comprising at least one of Li, Rb, Cs, Na, and K such as alkaline metal oxides, alkaline metal salts, e.g., Li2O, Cs2O, and LiAlO2.
Cathode 22 provides electrons. It may be transparent. Suitable materials for this layer include, e.g., Mg, Ca, Ag, Al, alloys of Ca and Mg, and ITO.
Polymer OLEDs may be made wherein a single layer of poly(phenylenevinylene) (PPV) or poly(2-methoxy-5-(2xe2x80x2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) functions as layers 16, 18, and 20.
Illustrative examples of known OEL device constructions would include molecularly doped polymer devices where charge carrying and/or emitting species are dispersed in-a polymer matrix (see J. Kido, xe2x80x9cOrganic Electroluminescent devices Based on Polymeric Materials,xe2x80x9d Trends in Polymer Science, 1994, 2, 350-355), conjugated polymer devices where layers of polymers such as poly(phenylenevinylene) (PPV) act as the charge carrying and emitting species (see J. J. M. Halls, D. R. Baigent, F. Cacialli, N. C. Greenham, R. H. Friend, S. C. Moratti, and A. B. Holmes, xe2x80x9cLight-emitting and Photoconductive Diodes Fabricated with Conjugated Polymers,xe2x80x9d Thin Solid Films, 1996, 276, 13-20), vapor deposited small molecule heterostructure devices (see U.S. Pat. No. 5,061,569, incorporated by reference, and C. H. Chen, J. Shi, and C. W. Tang, xe2x80x9cRecent Developments in Molecular Organic Electroluminescent Materials,xe2x80x9d Macromolecular Symposia, 1997, 125, 1-48), light emitting electrochemical cells (see Q. Pei, Y. Yang, G. Yu, C. Zhang, and A. J. Heeger, xe2x80x9cPolymer Light-Emitting Electrochemical Cells: In Situ Formation of a Light-Emitting p-n Junction,xe2x80x9d Journal of the American Chemical Society, 1996, 118, 3922-3929), and vertically stacked organic light-emitting diodes capable of emitting light of multiple wavelengths (see U.S. Pat. No. 5,707,745, incorporated by reference and Z. Shen, P. E. Burrows, V. Bulovic, S. R. Forrest, and M. E. Thompson xe2x80x9cThree-Color, Tunable, Organic Light-Emitting Devices,xe2x80x9d Science, 1997, 276, 2009-2011).
The present invention relates to adding a buffer layer, comprising a self-doped polymer, adjacent to an electrode layer in an organic electronic device. The invention further relates to adding a buffer layer, comprising an intrinsically conducting polymer having no mobile counterions, adjacent to an electrode layer in a small molecule, molecularly doped polymer, or conjugated polymer organic light emitting diode. For example, a buffer layer may be added between the anode layer and hole transporting layer of an organic electronic device to increase performance reliability. A buffer layer could also be added between a substrate and cathode layer.
When the buffer layer of the present invention is used in an organic electronic device such as an organic light emitting diode (OLED), the benefits to performance reliability include reducing or eliminating performance failures such as electrical shorts and non-radiative regions (dark spots). Typical performance failures are described in Antoniadas, H., et al., xe2x80x9cFailure Modes in Vapor-Deposited Organic LEDs,xe2x80x9d Macromol. Symp., 125, 59-67 (1997). The performance reliability of OLEDs can be influenced by a number of factors. For example, defects in, particles on, and general variations in the morphology at the surface of the materials comprising the substrate and electrode layers can cause or exacerbate performance failures that can occur in OLEDs. Particles or defects on the surface of the substrate or electrode layer may prevent the electrode surface from being coated uniformly during the deposition process. This can cause shadowed regions close to the particle or defect. Shadowed areas provide pathways for water, oxygen, and other detrimental agents to come into contact with and degrade the various lamp layers. This degradation can lead to dark spots which can grow into larger and larger non-emissive regions. This degradation can lead to immediate device failure due to electrical shorting or slower, indirect failure caused by interaction of the OLED layers with the atmosphere. The planarization provided by a conformal buffer layer can mitigate these imperfections.
Adding a buffer layer comprising an externally-doped polymer to an organic electronic device can cause an undesirable increase in operating voltage over time. This phenomenon is shown in U.S. Pat. No. 5,719,467, FIG. 5, incorporated by reference. However, the present inventors discovered that using a conducting self-doped polymer instead of an externally-doped polymer in the buffer layer does not increase the operating voltage, while still providing the benefits of a buffer layer.
One aspect of the invention features an organic electronic device having a buffer layer comprised of a self-doped conducting polymer, preferably a self-doped conductive polyaniline such as a conductive polyaniline incorporating sulfonic acid groups in the backbone.
The organic electronic device may be an organic light emitting diode (OLED). The OLED may have a transparent, semi-transparent, or opaque anode and/or cathode layer, depending on the desired direction of light emission from the OLED. Other aspects of the present invention feature a small-molecule, molecularly doped polymer, or conjugated polymer organic light emitting diode comprising a transparent or semitransparent electrode layer adjacent to a buffer layer comprising an intrinsically conducting polymer having no mobile counterions.
The intrinsically conducting polymer may be, e.g., polypyrrole, polyaniline, polythiophene, polyacetylene, and their derivatives.
As used in this specification:
xe2x80x9cdopantxe2x80x9d means an additive used to modify the electrical conductivity of a polymer; for example, the imine nitrogen of a polyaniline molecule in its base form may be protonated upon exposure of the polyaniline to an acidic solution thereby converting the polyaniline to its conducting form; the acid providing the proton may be referred to as the dopant;
xe2x80x9cexternally dopedxe2x80x9d means a polymer is exposed to an added substance that can change the polymer""s electrical conductivity; for example, an acidic solution can provide a hydrogen ion to dope a polyaniline molecule and can concurrently provide a counterion that is ionically, but not covalently, bonded to the polymer molecule;
xe2x80x9cself-dopedxe2x80x9d means that the doping moiety is covalently bonded to the polymer being doped;
xe2x80x9cintrinsically conductingxe2x80x9d means an organic polymer that contains polyconjugated bond systems and that can act as an electrical conductor in the absence of external conductive materials such as metal particles, carbon black, etc.;
xe2x80x9csmall-molecule OLEDxe2x80x9d means a multilayer heterostructure OLED having its non-polymer layers vapor deposited onto an electrode substrate in a vacuum chamber, wherein xe2x80x9cnon-polymerxe2x80x9d refers to low molecular weight discrete compounds that can be thermally vaporized without causing significant decomposition or other chemical changes; and
xe2x80x9cpolymer light emitting devicexe2x80x9d can include a molecularly doped polymer, conjugated polymer, or hybrid OLEDs, e.g., AlQ vapor deposited on top of solution coated PPV.
An advantage of at least one embodiment of the present invention is the reduction or elimination of mobile counterions in an organic electronic device. Preferably, counterion mobility is reduced or eliminated in the buffer layer of such a device. It is advantageous to immobilize these counterions because it is believed that they can migrate in the electrode structure and interfere with the movement of positive charges or electrons in the device.
Another advantage of at least one embodiment of the present invention is the avoidance of undesirable operating voltage increase over time.
Another advantage of at least one embodiment of the present invention is increased device lifetime and higher operating reliability.